Miniature variable-resistance device with flexible disk contact

ABSTRACT

A potentiometer comprising a body of homogeneous vitreous material in which a plurality of wire leads are embedded flush with a surface of the vitreous body which is received in a metal frame. A cup-shaped housing encloses the flush surface of the body and engages the frame while receiving a rotatable plug having a boss with a screwdriver slot extending through this cover. An arcuate contact strip is applied, e.g. by vacuum depositing to the surface while a thin deformable plate yieldably overlies the contact strip in spaced relation therewith, the contact strip being angularly fixed upon rotation in the plug, a spring-loaded ball presses a limited region of the disk against the contact strip to form the wiper.

United States Patent Tavzes et al.

[54] MINIATURE VARIABLE-RESISTANCE DEVICE WITH FLEXIBLE DISK CONTACT{72] Inventors: Radovan Tavzes; Jelko Koron; Evgen Kan- 21 Appl. No.:62,419

[30] Foreign Application Priority Data Aug. 11, I969 Yugoslavia..P-2069/69 [52} US. Cl ..338/154, 338/174 [51] Int. Cl ..H0lc 9/02 [58]Field ofSearch ..338/96,118,154,157,162l64,

[56] References Cited UNITED STATES PATENTS 7 3,516,041 6/1970 Estlick..338/96 X 1 Apr. 18, 1972 Primary Examiner-Laramie E. Askin AssistantExaminer-Gerald P. Tolin Att0rneyl(arl F. Ross 57 ABSTRACT Apotentiometer comprising a body of homogeneous vitreous material inwhich a plurality of wire leads are embedded flush with a surface of thevitreous body which is received in a metal frame. A cup-shaped housingencloses the flush surface of the body and engages the frame whilereceiving a rotatable plug having a boss with a screwdriver slotextending through this cover. An arcuate contact strip is applied, eg byvacuum depositing to the surface while a thin deformable plate yieldablyoverlies the contact strip in spaced relation therewith, the contactstrip being angularly fixed upon rotation in the plug, a spring-loadedball presses a limited region of the disk against the contact strip toform the wiper.

10 Claims, 3 Drawing Figures Patented April 18, 1972 'FIG.I

E Y WWW. RN T KK m V N mE win REE F l G 2 ATTORNEYMINIATURE'VARlABLE-RESISTANCEDEVICE WITH FLEXIBLE DISK CONTACT It is anobject of the present invention to provide a miniature thin-filmpotentiometer which is particularly suitable forv use in printedcircuits.

According to the invention, the mechanism of the miniature potentiometeris built into a can which is moistureand waterproof, welded or solderedto the header. The can protects the potentiometer mechanism againstexternal influences or damages and transmits the dissipated heat fromthe interior to the ambient. The can is electrically connected to themass lead in order to prevent the influence of I electromagneticperturbances on the resistance film of the potentiometer. Through anopening in the can there protrudes an extension of an insulating plugfor adjusting the potentiometer resistance. This extension has a slotfor a screw driver, or is adapted to receivean adjusting knob. Theheader consists of a metal ring intowhich wire leads are sealed with theaid of aglass or ceramic plate. One of the wire leads is welded to themetal ring for ground connection, the remaining wire leads are connectedto the ends of the resistance film and to the resilient contact plate.The materials for the header, i.e. glass or ceramic, metal rings andwire leads are so chosen that their coefficients of thermal expansionare matched in a broad temperature range of use. Moreover, the glassorceramic must meet severe requirements regarding vacuumdepositing ofmetal segments and the resistance film.

According v to present invention the resistance film is vacuum-depositeddirectly upon the polished upper surface of the glass or ceramicssubstrate. After being sealed, the wire leads are worked together withthe glass or ceramics surface so that the entire upper surface of thesubstrate is highly smooth and even and that the ends of the wire leadsare flush with the upper surface of the substrate. The resistance filmhas the shape of a cut-out ring or a circular horseshoe and is sooriented that the ends of the film are connectedto the wire leads. Theseconnections are made with the aid of metal segments vacuum-deposited onthe ends of the wire leads and the surrounding surface of the substrate.In this way a better reliability of the connections of the wire can beobtained.

The resistance film is made of one or more layers. Its design dependsupon the required value of the resistance and other properties required,such as the temperature coefficient of resistance, mechanical stability,contact properties and other. The single-layer film is usually made ofametal alloy, e.g. Ni- Cr, and can be used for resistances of up to1,000 ohms. In the case of a two-layer film, the lower layer is made ofa material with a predominantly negative temperaturecoefficient ofresistance, i.e. of the semiconductor type, such as are combinations ofoxides and metals, e.g. cermets; the upper layer is of a metal having aprevailingly positive temperature coefficient of resistance. This uppermetal layer is continuous for resistance ranges between 1 and 100kiloohms, while for higher resistance values the layer is mosaic-shapedand has a high layer resistance.

in the middle of the upper substrate surface, a conductive metal layeris also vacuum-deposited and connected to the central wire lead. Alittle round resilient contact plate is concentrically fixed to thislayer. This plate is made of a resilient metal with a durable contactsurface. This small plate can-be fastened by ultrasonic soldering or bygluing with a conductive adhesive. This small plate is so shaped thatits edge overlaps the resistance film and touches it only in that pointwhere a spring lever of the tumable plug, preferably having the shape ofa steel ball and a helicoidal spring, which are placed in an eccentricgroove of the tumable plug, presses against the resistance film. Apressure on the plate creates an electrical contact between the selectedpoint of the resistance film and the central wire lead through saidplate and the central vacuumdeposited conductive layer. It followstherefrom that the resilient plate and the spring-loaded eccentric metalball pressing the plate against the resistance film, perform thefunction of a wiper. The resilient'plate prevents wear of the resistancefilm because it does not slide upon the resistance film, but it onlyadheres to it when it is pressed. In this way a good contact is reachedwith the resistance film without risk of wear and tear or damage, aswell as without displeasing noise and crepitation when the potentiometerresistance is being adjusted.

In the middle of its upper part, the tumable insulating plug has anextension protruding through the can and faciliating turning of the plugand thus the variation of the potentiometer resistance. A ring-shapedgasket embraces the extension and provides for vacuum tightness betweenthe can and the plug. Further, the plug comprises a tooth for thelimitation of the turning angle so that the plug is allowed to turn onlyby a determined angle. On its lower side, the plug has two bores intowhich the helicoidal spring-loaded metal balls are placed; theeccentrically positioned ball presses the resilient contact plateagainst the selected point of the resistance film. The can ishermetically closed by welding or soldering to the metal ring of theheader.

For depositing metal segments, any vacuum procedure is suited whichprovides for a quick deposition of a solid and homogeneous thin metallayer having a low surface resistance and a good adhesiveness to thesubstrate, preferentially evaporation from a boat or a helix, flashevaporation or sputten'ng. The procedure mentioned requires a vacuum ofat least 10 3 torr. The surface to which the metal segment should beapplied must be thoroughly cleaned and heated. During the procedure,-the header temperature should be maintained constant and enough high,the speed of deposition and the composition of the deposited materialshould also be kept constant. For depositing resistance films proceduresare suitable which allow for the creation of highly homogeneous andstrictly defined thin layers so by the structure as well as by thecomposition and by electric properties. Before all, we can use flashevaporation, sublimation and sputtering. Still more severe requirementsthan for depositing metal segments apply for depositing resistancefilms.

During deposition and composition of the layer deposited on thesubstrate can be varied from a metallic substance throughout tosemiconductor materials simply by adding reactive gases to the residualatmosphere. The sequence of metal and/or semiconductor layers results inhigh quality resistance film for high resistance with a low temperaturecoeflicient. Sputtering, however, also allows for depositing substanceswhich otherwise are evaporated with difiiculty.

By introducing a reactive gas also in this procedure, the deposition ofsubsequent layers of different compositions may be controlled similarlyas in the case of evaporation.

Thin layers deposited can be artificially stabilized by continuous orcyclic heating in air, vacuum, or under a protective atmosphere. A veryusual procedure for the stabilization of layers consists in a longstorage of headers with resistance film deposited at an increasedtemperature. Before all, it is important to obtain the stabilization ofelectrical properties of the resistance film.

The potentiometer according to the invention differs from knownembodiments by the following essential advantages. The resistance layeris extremely even and smooth warranting for a very low-noise level whensetting the value of resistance. Due to smoothness of the film anytransfer of material between the contact plate and the film is avoided.The smoothness of the film and the special design of the wiper by thesmall contact plate described also allows for a high contact pressureresulting in a reliable contact so that the potentiometer is resistantagainst vibrations and mechanical shocks. This design gives a longuseful life of the potentiometer according to the invention. Thin layersevaporated in differing compositions result in a very low temperaturecoefficient ofresistance, the relatively short resistance film has a lowinductance particularly when compared with wire-wound potentiometers. Inaddition the stray capacitance is low because the resistance trace isdistant also from metal parts.

An embodiment of the miniature potentiometer according to the presentinvention will be described in detail with reference to the accompanyingdrawing, in which FIG. 1 shows a longitudinal cross-section of thepotentiometer,

FIG. 2 shows a plan view of the potentiometer according to FIG. 1, and

FIG. 3 shows a plan view of a header with deposited contact segments andthe resistance film.

The miniature potentiometer according to FIGS. 1 and 2 has a metal can 1equal to the standard cans for transistors of type TO 5.

Wire leads 3, 4, 5 are sealed into a metal ring 2 with the aid ofhomogeneous glass, while the ground lead 6 is welded to the metal ring2. The upper surface of the glass substrate 7 is evenly ground andoptically polished. The resistance film 8 having the shape of a cut-outring of a thin layer of resistive substance is then deposited on saidupper surface. The ends of the resistance film 8 are electricallyconnected to wire leads 3, 5. A good contact between the ends of theresistance film 8 and the wire leads 3, 5 is warranted by metal contactsegments 8 shown in FIG. 3. Usually the contact segments 8' aredeposited below the resistance film 8. In some cases it is more suitableto design the contact segments in the form of two layers, one beingdeposited below the resistance film and the other layer above it. In themiddle of the upper surface of the glass substrate 7 and separated fromthe resistance film 8, a contact metal segment 8" is deposited. A smallround resilient plate 9 is soldered to the segment mentioned, whichwarrants for a good electrical connection between the plate 9 and thewire lead 4. The middle part of plate 9 is impressed so that the platebecomes the shape of a saucer with a round even bottom and a ringparallel to the bottom and raised above the bottom plane. The surface ofsaid small plate is smooth, corrosionproof and has good contactproperties. This plate is made of resilient copper and is gold-plated.The bottom of the small The resistance film and metal contact segmentsare vacuum-deposited onto the glass substrate with the aid of adequatelyshaped screening masks. The resistance film has a thickness of about 0.1um, whereby it fully adheres to the polished surface and is highlysmooth. The film is hard, strong and firmly adhering to the substrate.The electrical and physical properties are artificially stabilized withtime. Moreover, the resistance film has a low temperature coefficient ofelectric resistance and a surface with good contact properties.Depending upon the desired resistance value, the resistance film is madeof different materials and. with various structures. Low-resistancefilms up to about 1,000 ohms are made preferentially of metal alloys ofa homogeneous composition contact plate 9 is firmly soldered to thecontact segment 8".

Thereby this plate is stiffly connected to the glass substrate 7concentrically with the resistance film 8. The edge of this plateprotrudes over the outer border of the resistance film and does nottouch it when no pressure is applied to the plate. To the glasssubstrate 7 with-the metal ring 2, a metal can 1 is fixed containing aturnable plug 10, a gasket 11, two helicoidal springs 12 and 12' and twoballs 13 and 13'. The upper surface of can 1 is provided with a centralround opening, through which protrudes a concentric extension 10' forturning the plug 10 by two helicoidal springs 12 and 12' and two balls13 and 13. The upper surface of can 1 is provided with a central roundopening, through which protrudes a concentric extension 10' for turningthe plug 10 with the aid of a slot 14. An indent 15 for the limitationof turning the plug 10 is impressed on the edge of the can. Aring-shaped gasket is inserted between the plug 10 and the top of thecan, hermetically separating the interior of the potentiometer from theambient atmosphere. The gasket is made of a heat-resistant elastomer.The plug 10 is made of insulating and heat-resistant material, havingthe shape of a low cylinder with obliquely cut bottom.

On the upper side, the plug 10 comprises a tooth 16 on its border forlimiting the rotation. On the bottom side, the plug 10 also comprisestwo bores 17 and 17. The bore 17 is placed in the center of the plug andis stepped. The bore 17' is eccentrically positioned. Into the bores 17,17 two helicoidal springs l2, l2 and two steel balls 13 and 13' areinserted. The spring 12 presses the plug 10 against the can 1 and theball 13 against the bottom of contact plate 9. The spring 12 presseswith the aid of ball 13' the small contact plate 9 onto the resistancefilm 8, thereby establishing an electric contact between the resilientcontact plate and the resistance film. By turning the plug by 270, theresistance film can be moved. A step in the central bore 17 of plug 10protects the contact plate, because with an excessive external pressurethe ball 13 rests on the step, thereby preventing that the plug 10should touch the plate 9.

throughout. The films up to about 50 kiloohms are made of two or severallayers; the lower layer is of a semiconductor type, e.g. cermet SiO Cr,or Cr Ce o and similar, and covered by an upper metal layer. The uppermetal layer reduces the temperature coefficient of electric resistanceand improves contact properties of the resistance film. For still higherresistance values the film is a multi-layer film, but the upper metallayer is composed of single separated mosaic-like metal islets so thatit is not continuous.

For still higher resistance values, the resistance film consists ofseveral layers with continuous or sudden changes in its compositionand/or structure. The interior of the potentiometer is hermeticallyseparated by the gasket 11 on one side, and on the other side by thelower edge of can 1 being hermetically sealed to the metal ring 2 of theheader.

We claim:

1. A miniature variable resistance device comprising a body ofhomogeneous vitreous material. having a ground opticallypolishedsurface; a plurality of conductive leads embedded in said body andterminating flush with said surface; an arcuate contact stripvacuum-deposited upon said surface and conductively connected with saidleads; a resiliently deflectible conductive disk overlying said surfaceand normally spaced above said strip while being angularly fixed withrespect to said body; and a rotatable member disposed above said surfaceand provided with an element bearing upon said disk and deflecting sameinto engagement with said strip, said disk being electrically connectedwith one of said leads.

2. The device defined in claim 1 wherein said element includes a ballmounted in said member for movement toward and away from said surfaceand a helicoidal spring in said member resiliently biasing said ballagainst said disk over a limited region of the latter.

3. The device defined in claim 2, further comprising a cupshaped cansurrounding said surface and provided with a central opening; saidmember being formed as a plug rotatably received in said can and havinga boss extending into said opening for rotation of said member.

4. The device defined in claim 3, further comprising a sealing gasketsurrounding said opening and bearing upon said member around said boss.

5. The device defined in claim 4, further comprising mutually engageableformations on said can and on said member for limiting rotation of saidmember within said can.

6. The device defined in claim 1 wherein said strip is a resistance filmof homogeneous composition.

7. The device defined in claim 1 wherein said strip consists of ametallic layer and a semi-conductor layer disposed one above the other.

8. The device defined in claim 1 wherein said strip is formed withdiscrete mosaic-like metal islets.

9.. The device defined in claim 1, further comprising vacuum-depositedmetallic contact segments electrically connecting said strip with saidleads.

10. The device defined in claim 1 wherein said strip is a circular arcsegment connected at its ends to two of said leads, said potentiometerfurther comprising a central contact layer deposited on said surface andelectrically connected to another of said leads between the ends of saidstrip, said disk being dished and having a convex portion bearing uponsaid layer, said member being constituted as a rotatable plug having acentral pivot ball seated in said convex portion and,

.limiting rotation of said plug within said can, said element includinga spring-loaded ball carried by said plug and bearing against said disk,and a frame in conductive relationship with one of said leads andsurrounding said body and engaged by said can, said body being composedof glass.

iii!

1. A miniature variable resistance device comprising a body ofhomogeneous vitreous material having a ground optically-polishedsurface; a plurality of conductive leads embedded in said body andterminating flush with said surface; an arcuate contact stripvacuum-deposited upon said surface and conductively connected with saidleads; a resiliently deflectible conductive disk overlying said surfaceand normally spaced above said strip while being angularly fixed withrespect to said body; and a rotatable member disposed above said surfaceand provided with an element bearing upon said disk and deflecting sameinto engagement with said strip, said disk being electrically connectedwith one of said leads.
 2. The device defined in claim 1 wherein saidelement includes a ball mounted in said member for movement toward andaway from said surface and a helicoidal spring in said memberresiliently biasing said ball against said disk over a limited region ofthe latter.
 3. The device defined in claim 2, further comprising acup-shaped can surrounding said surface and provided with a centralopening; said member being formed as a plug rotatably received in saidcan and having a boss extending into said opening for rotation of saidmember.
 4. The device defined in claim 3, further comprising a sealinggasket surrounding said opening and bearing upon said member around saidboss.
 5. The device defined in claim 4, further comprising mutuallyengageable formations on said can and on said member for limitingrotation of said member within said can.
 6. The device defined in claim1 wherein said strip is a resistance film of homogeneous composition. 7.The device defined in claim 1 wherein said strip consists of a metalliclayer and a semi-conductor layer disposed one above the other.
 8. Thedevice defined in claim 1 wherein said strip is formed with discretemosaic-like metal islets.
 9. The device defined in claim 1, furthercomprising vacuum-deposited metallic contact segments electricallyconnecting said strip with said leads.
 10. The device defined in claim 1wherein said strip is a circular arc segment connected at its ends totwo of said leads, said potentiometer further comprising a centralcontact layer deposited on said surface and electrically connected toanother of said leads between the ends of said strip, said disk beingdished and having a convex portion bearing upon said layer, said memberbeing constituted as a rotatable plug having a central pivot ball seatedin said convex portion and spring loaded thereagainst, a can surroundingsaid plug and provided with a central opening, a gasket surrounding saidopening and sealingly engaging said plug, said plug having a bossprovided with a screw driver slot projecting through said opening,cooperating formations on said can and said plug for limiting rotationof said plug within said can, said element including a spring-loadedball carried by said plug and bearing against said disk, and a frame inconductive relationship with one of said leads and surrounding said bodyand engaged by said can, said body being composed of glass.